When following the documentation for updating J!3 -> 4 -> 5 -> 6 somewhere along the line the plg_fields_repeatable (written for J3.9 isn't updated. Core file (J!3) but not updated and left behind in the system. So when disabling all 3th party extensions this extension is left enabled, causing the DISabling of backwards compatibility plugin to fail on a core component.

FIX:

File: repeatable.php

replace the JLoader::import() call with a proper use statement and add the missing FORM import:

`
-public function onCustomFieldsPrepareDom($field, DOMElement $parent, JForm $form)

+public function onCustomFieldsPrepareDom($field, DOMElement $parent, Form $form)`

After this change, the backwards comp. plugins can safely be disabled and the update to J!6 can continue.